Steel alloy



United States Patent 3,168,397 STEEL ALLOY Lawrence R. Scharfstein, Reading, Pa., assignor to The Carpenter Steel Company, Reading, Pa, a corporation of New Jersey No Drawing. Filed Jan. 3, 1962, Ser. No. 164,140 6 Claims. (Cl. 75-128) This invention relates to corrosion resistant steel alloys and more particularly to a steel alloy especially well suited for fabricating parts which in addition to being resistant to corrosion in a medium containing sulfuric acid are also capable of sustaining stress in such a medium without cracking.

A corrosion resistant alloy is generally identified as one which in a given medium will not lose metal or be penetrated at a rate greater than would be tolerable when fabricated into parts. For example, a composition and parts made therefrom having a corrosion rate of about 0.005 inch penetration per year in 10% by weight boiling sulfuric acid are found in practice eminently well suited for use where resistance to hot sulfuric acid is required.

Quite unlike the aforementioned phenomenon of corrosion resistance, failure due to cracking of a part when subjected to stress in a corrosive medium is not accompanied by any appreciable loss in metal. In fact, the amount of metal lost may be so slight as to be undetectable except with the most sensitive techniques. Stress corrosion cracking may occur in a particular medium when a part is subjected for a relatively short time to a stress which is much less than the maximum load the part could sustain without fracturing in a noncorrosive atmosphere or even in an atmosphere containing a different corrosive agent.

The precise mechanism which results in the failure of a part because of stress corrosion cracking in a particular medium is not completely known or understood. However, it has been established that while a part made from a particular steel alloy may have a useful life measured in years or tens of years in a medium containing sulfuric acid so long as it is not subjected to stress, the same part may fail in a matter of hours when a surface thereof is under tension while exposed to attack by sulfuric acid.

The present invention stems from my discovery that when the elements nickel, chromium, molybdenum, copper and iron are combined in critically controlled proportions, a composition is provided which, in addition to being resistant to corrosion, is capable of sustaining stress in a medium containing sulfuric acid without cracking.

It is, therefore, an object of the present invention to provide a steel alloy having improved resistance to stress corrosion cracking. It is another object of this invention to provide such an alloy having improved stress corrosion resistance which is also hot workable, ductile, and easy to fabricate. It is a more specific object of this invention to provide such an alloy which is relatively inexpensive, which has a homogeneous austenitic microstructure after being hot worked or welded and which is completely free from stress corrosion cracking when immersed in a sul furic acid medium. Other objects will be apparent to those skilled in the art from reading the following specification.

I have discovered that the foregoing objects can be achieved with a steel containing chromium, nickel, molybdenum and copper in the following proportions:

Excessive carbon detracts from the corrosion resistance of my composition and is, therefore, limited to no more than 0.06%. Phosphorus is limited to residual amounts, no more than about 0.03% and the sulfur content is preferably kept below about 0.015%.

Usually, my composition contains from about 0.025% to 0.035% nitrogen although as much as 0.05% or more nitrogen may be present, if desired.

Preferably, controlled amounts of titanium or columbium are included in my composition in order to stabilize the composition as an essentially homogeneous, austenitic alloy and to prevent the precipitation of chromium car bides, particularly when articles made from the composition are welded. For this purpose, up to 2% titanium or up to 1% columbium is present. I preferably utilize a minimum amount of titanium equal to four times the carbon content, while in the case of columbium, I preferably utilize a minimum amount equal to eight times the carbon content. In keeping with the usual commercial practice, columbium is accompanied by some tantalum and therefore, the percent stated for columbium is to be understood as including tantalum in the usual proportion.

Additions of misch metal or boron improve the hot workability of my composition. An addition of misch metal in an amount suflicient to result in retention of from about 0.10% to 0.30% misch metal in the solidified metal provides improved hot workability Without adversely affecting the corrosion resistance of the alloy. Instead of misch metal, I also utilize boron in amounts up to about 0.01% to improve the hot workability of my composition because with boron present instead of misch metal, the alloy is characterized by less non-metallic inclusions which assists in obtaining better corrosion resistance. Preferably, the alloy contains from about 0.003% to 0.007% boron and best results are achieved with a boron content of from 0.003% to 0.005%.

The remainder of the composition is essentially iron which is intended to include such impurities as are consistent with good commercial practice as well as such additional elements which are in keeping with good metallurgical practice and do not impair the desirable properties of the composition. For example, manganese is included in an amount up to about 2% to prevent hot shortness. Silicon is included for the purpose of deoxidizing and cleaning the melt. When present in amounts above about 1%, silicon adversely affects the corrosion resistance and forgeability of the composition. Preferably, silicon is present in an amount ranging from about 0.40% to 0.75%.

Thus, my preferred composition consists essentially of:

Percent Carbon Up to 0.06 max. Manganese Up to 2 max. Silicon 4.40-0.75. Phosphorus Up to 0.03 max. Sulfur Up to 0.015 max.

elements in the amounts normally present as im purities in such steels.

1 Columbium is used in a minimum amount of 8 times the carbon contentand when titanium is included instead of coludmbium, a minimum of 4 times the carbon content is use Misch metal (primarily made up of cerium and 1antha num) or boron is included in the ranges indicated.

In my composition it is essential that the elements chromium, nickel, molybdenum and copper be present in the minimum amounts stated for the composition to have the required corrosion resistance, resistance to corrosion in sulfuric acid, resistance to pitting and the required freedom from stress corrosion cracking in sulfuric acid. Thus, it is essential for the attainment of all these properties that there be a minimum chromium content of 17.5%, a minimum nickel content of 32.5%, a minimum molybdenum content of 2% and a minimum copper content of 3%.

Chromium may be present in my composition .in amounts ranging up to about 26%. Above about 26%, chromium causes the formation of an undesirable double phase microstructure. Thus, while the larger chromium contents tend to increase the general corrosion resistance of the composition, best results are achieved with chromium ranging from about 20% to 22%.

When molybdenum is present in an amount greater than 3%, there is an adverse effect upon the strength and the stress corrosion resistance of the composition because of .the formation of a sigma phase. Best results are achieved with a molybdenum content of from 2% to 2.5%.

It is essential that all of the copper present he retained in solid solution in my composition, and for this reason it is necessary to limit the copper content to about 4% because above that amount some of the copper may be precipitated out ofsolid solution, particularly when the composition is heated at an elevated temperature for more than a very short period.

In my composition, the nickel content must beat least 32.5% in order to achieve the desired freedom from stress corrosion cracking in sulfuric acid. Thus, with nickel present in the stated amounts of from 32.5% to about the desired properties including freedom from stress corrosion cracking in sulfuric acid are achieved. Greater amounts of nickel than 35% have no beneficial eifect upon the resistance of my composition to corrosion and stress corrosion cracking in sulfuric acid and only add unnecessarily to its cost;

' The following examples of my composition, unless otherwise indicated, were processed as follows. The heats were melted and cast in the usual manner into ingots from which bars were forged. The bars were readily machined and annealed toprovide specimens i /s inch wide, A2 inch thick and 3% inches long. Holes were drilled in each specimen near its extreme ends and each was then deformed by bending it through its thinnest dimension around a mandrel having an outside diameter of 1 inch. Each specimen was thus formed into a U-shaped test specimen having an arc of about 180 with substantially parallel straight segments of substantially equal length extending from the ends of the arc. A bolt wrapped with Teflon tape was inserted through the. holes of each specimen and a nut was tightened on the bolt to maintain the specimen under stress in the shape of a U-bend. The concave inner surface of each specimen was thus placed under compression and the outside surface of the bend under tension, the highest stress occurring in the outer fibers. The specimens thus prepared and stressed were immersed in non-aerated, boiling 30 to 40% sulfuric acid.

As is customary, percent concentrations of elements in the following examples and throughout this specification refer to percent by weight.

Example 1.-An alloy was prepared having the following analysis:

Percent Carbon 0.054.

Manganese 0.91. Silicon 0.51. Phosphorus 0.012. Sulfur 0.008. Chromium 20.57. Nickel 32.68. Molybdenum 2.32. Copper 3.28. Columbium (+tantalum) 0.81. Boron 0.006.

Nitrogen 0.039. Iron Balance, except for other elements in the amounts normally found as impurities in such steels.

Five U-bend test specimens of this alloy were found to be entirely free of cracks after being kept in the boiling 30-40% sulfuric acid for 330 hours. It is to be noted that such U-bend specimens have a complex stress pattern varying from high to low stress levels and, having been cold worked, are ideally suited for testing the susceptibility of the alloy to stress corrosion cracking.

Example 2.An alloy was prepared having the following analysis:

Percent Carbon 0.057.

Manganese 0.95. Silicon 0.52.

Phosphorus 0.010. Sulfur 0.008. Chromium 20.18. Nickel 33.47. Molybdenum 2.29. Copper 3.33. Columbium tantalum) 0.81. Boron 0.006. Nitrogen 0.041. Iron Balance, except for impurities. As in the case of the specimens formed from the alloy of Example 1, five U-bend specimens of the alloy of Example 2 when tested in the same manner in 30-40% boiling sulfuric acid for 330 hours,v were found to be entirely free of cracks.

Examplei-An alloy was prepared having the following analysis:

for impurities.

Five specimens of this alloy were also found to be entirely free of cracks after 330 hours in 30-40% boiling sulfuric acid.

Example 4.An alloy was prepared having the following analysis:

Percent Carbon 0.028.

Manganese 0.94. Silicon 0.66.

Phosphorus 0.014. Sulfur 0.011.

Chromium 20.00.

Nickel 35.27.

Molybdenum 2.97. Copper 3.27. Columbium tantalum) 0.54. Nitrogen 0.115. Misch metal (Not tested.) Iron Balance, except for impurities.

Six specimens of this alloy were found to be entirely free of cracks when the U-bend specimens were removed from the boiling 30-40% sulfuric acid after 290 hours.

The specimens formed from the alloys of Examples 1-4 each had a homogeneous, single phase austenitic microstructure. Articles made from my composition may be readily formed and when welded retain a homogeneous, single phase microstructure.

In contrast to the complete resistance to stress corro- O sion cracking in the boiling 30-40% sulfuric acid demonstrated by the U-bend specimens of Examples 1-4, are the following results. An alloy was prepared as was described in connection with Example 1 but having the folfor impurities.

It will be noted that except for the relatively small reduction in the nickel content below the critical minimum level for this element, this alloy is substantially identical to the alloys of Examples 1-4. Six U-bend specimens, prepared from this alloy in exactly the same manner described herein above in connection with the specimens formed of Examples l-4, were immersed in the boiling 30-40% sulfuric acid and each of these six U-bend specimens cracked before they were removed. One of the specimens was found cracked after only 24 hours in the boiling sulfuric acid While the last of the six specimens to crack did so in less than 135 hours. It may be noted that the cracks in each of these six specimens appeared on the outer surface, that under tension, apparently beginning at an edge slightly away from the center of the arc of the U-bend. No cracks were found on the inner, compression surface of the specimens.

While my composition is suitable for use under conditions where it will be exposed to attack by a variety of corrosive agents, it is especially well suited for use where, among other things, it will be exposed to sulfuric acid. My composition is characterized by outstanding resistance to corrosion by sulfuric acid and is completely free from 1. A corrosion resistant austenitic steel alloy, consisting essentially of in percent by weight:

From 17.5 :to 26% chromium, From 32.5% to 35 nickel, From 2% to 3 molybdenum, From 3 to 4% copper,

within the tolerances of good melting practice, and the remainder essentially iron, said alloy being characterized by freedom from stress corrosion cracking when under stress while exposed to sulfuric acid.

2. A homogeneous corrosion resistant austenitic steel alloy consisting essentially of in percent by weight within the tolerances of good melting practice:

Up to 0.06% carbon,

Up to 2% manganese,

Up to 1% silicon,

Up to 0.03 phosphorus,

Up to 0.015% sulfur,

From 17 .5% to 26% chromium, From 32.5 to 35 nickel, From 2% .to 3 molybdenum, From 3 to 4% copper,

Up to 0.05% nitrogen,

Up to 1% columbiurn,

Up to 0.3 misch metal,

colurnb-ium when present being in an amount equal to at least eight times the carbon content, the remainder being essentially iron, and said alloy being characterized by freedom from stress corrosion cracking when under stress while exposed to sulfuric acid.

3. A homogeneous corrosion resistant austenitic steel alloy consisting essentially of in percent by weight within the tolerances of good melting pnactice:

Up to 0.06% carbon,

Up to 2% manganese,

Up to 1% silicon,

Up to 0.03 phosphorus,

Up to 0.015% sulfur,

From 17.5% [to 26% chromium, From 32.5% to 35% nickel, Fnom 2% to 3% molybdenum, From 3 to 4% copper,

Up to 0.05% nitrogen,

Up to 2% titanium,

Up to 0.3 misch metal,

titanium when present being in an amount equal to at least four times the carbon content, the remainder being essentially iron, and said \alloy being characterized by freedom from stress corrosion cnacking when under stress while exposed to sulfuric acid.

4. A homogeneous corrosion resistant austenitic steel alloy consisting essentially of in percent by weight within the tolenances of good melting practice:

Up to 0.06% carbon,

Up to 2% manganese,

Up to 1% silicon,

Up to 0.03 phosphorus,

Up to 0.015 sulfur,

From 17.5% to 26% chromium, From 32.5% to 35 nickel,

7 From 2% to 3 molybdenum, From 3% to 4% copper, Up to 0.05% nitrogen, Up to 1% columbium, Up to 0.01% boron,

columbium when present being in an amount equal to at least eight timm the carbon content, the remainder being essentially iron, and said alloy being characterized by freedom from stress corrosion cracking when under stress while exposed to sulfuric acid.

5. A homogeneous corrosion resistant austenitic steel alloy consisting essentially of in percent by weight within the tolerances of good melting practice:

Up to 0.06% carbon,

Up to 2% manganese,

Up \to 1% silicon,

Up to 0.03 phosphorus,

Up to 0.015% sulfur,

From 17.5% to 26% chromium, From 3 2.5 to 3 5 nickel, From 2% to 3 molybdenum, From 3 to 4% copper,

Up to 0.05% nitrogen,

Up to 2% titanium,

Up to 0.01% boron,

titanium when present being in an amount equal to at least four times the carbon content, the remainder being essentially iron, and said alloy being characterized by freedom from stress corrosion cracking when under stress While exposed to sulfuric acid.

6. A homogeneous corrosion resistant. austenitic steel alloy consisting essentially of in percent by Weight Within the tolerances of good melting practice:

Up to 0.06% carbon,

Up to 2% manganese,

From 0.4% to 0.75% silicon,

Up to 0.03 phosphorus,

Up to 0.015 sulfur,

From 20% to 22% chromium, From 32.5% to nickel, From 2% 1102.5 molybdenum, From 3% to 4 copper,

At least 0.025 nitrogen,

' References Cited in the file of this patent UNITED STATES PATENTS 1,528,478 Hadfield Mar. 3, 1925 2,214,128 Font-aria Sept. 10, 1940 2,398,702 Fleischmann Apr. 16, 1946 2,423,665 Sullivan et a1 July 8, 1947 2,553,330 Post et a1 -1 May 15, 1951 2,857,266 Anger Oct. 21, 1958- 2',879,194 Eichelberger Mar. 24. 1959 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 ,168 ,397 February 2, 1965 Lawrence R. Scharfstein It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2 line 63 opposite "Silicon", for "4 40-0 .75 1 read 0.40-0.75. column 5, lines 17 and 18, for

Nitrogen d Misch metal Misch metal Tea Nitrogen Signed and sealed this 20th day of July 1965.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

1. A CORROSION RESISTANT AUSTENITIC STEEL ALLOY, CONSISTING ESSENTIALLY OF IN PERCENT BY WEIGHT: FROM 17.5% TO 26% CHROMIUM, FROM 32.5% TO 35% NICKEL, FROM 2% TO 3% MOLYBDENUM, FROM 3% TO 4% COPPER, WITHIN THE TOLERANCES OF GOOD MELTING PRACTICE, AND THE REMAINDER ESSENTIALLY IRON, SAID ALLOY BEING CHARACTERIZED BY FREEDOM FROM STRESS CORROSION CRACKING WHEN UNDER STRESS WHILE EXPOSED TO SULFURIC ACID. 